(1) Field of the Invention
The present invention relates to a multi-layer woven fabric comprising a plurality of woven fabric layers and having a three-dimensional structure suitable as a reinforcing fiber for a fiber-reinforced composite material, and to a composite material comprising the multi-layer woven fabric as a reinforcer.
More specifically, the present invention relates to a multi-layer woven fabric in which honeycomb-like cells can be formed by a specific combination of combined portions and non-combined portions when the woven fabric is expanded, i.e., opened out, and to a high-grade composite material having excellent mechanical characteristics, which is obtained by combining this multi-layer woven fabric with a specific resin.
(2) Description of the Related Art
As one conventional composite material, there is known a structural material formed by bonding a surface member forming a surface layer to a core material having honeycomb-like structure (hereinafter referred to as "honeycomb core").
In general, conventional honeycomb cores are obtained by coating an adhesive in stripes spaced equidistantly on a thin sheet such as a paper, an aluminum foil or a film, laminating and bonding such adhesive-coated thin sheets, and expanding the bonded structure to form honeycomb-like structure having a multiplicity of cells.
It is known that a plane woven fabric composed of glass fibers or the like is used as the sheet material for forming a honeycomb core according to the abovementioned process, and it is also known that a composite material is prepared by impregnating this honeycomb core with a thermosetting resin such as an epoxy resin. However, this honeycomb core does not have a sufficient tensile strength, peel strength and shear strength of the bonded surfaces. Although the use of a honeycomb structural material as a structural material of an aircraft is now desired, a satisfactory honeycomb structure has not been obtained because of the abovementioned defect.
U.S. Pat. No. 3,102,559 discloses a composite material formed by impregnating a honeycomb structure woven from yarns composed of natural fibers, nylon fibers, glass fibers or the like with a thermosetting resin. In this composite material, the tensile strength of the bonded surfaces is improved and a relatively high compression strength is attained because the weaving honeycomb structure is combined with the thermosetting resin. However, this composite material is still unsatisfactory as a structural material for an aircraft, and since the composite material is brittle, if the stress is imposed repeatedly, the composite material is liable to be broken.
Furthermore, a composite material is known which comprises a mat of carbon fibers or aramid fibers impregnated with a thermosetting resin. Although this composite has a high tensile strength and an excellent compression strength, the composite material is brittle and still has an insufficient impact strength. Accordingly, application of the composite material to fields where the conditions are more severe than in the conventional fields, for example, application to the field of aircraft, is difficult, and the application range of the composite material is limited. A light weight is an important condition for application to the field of aircraft. In this composite material, if it is intended to decrease the weight, the tensile strength and compression strength must be reduced, and when stress is imposed repeatedly, the composite material is liable to be broken and the impact resistance degraded. Moreover, the composite material exhibits a poor durability and heat resistance, when an aircraft part is repeatedly exposed to a high temperature and a low temperature.